CLASSIFICA TION OF MARSUPIA LS. 687 



position. The genital ducts of the females are often 

 separate throughout, so that there are two uteri and two 

 vaginae. But the bent proximal parts of the vaginas some- 

 times fuse and form a caecum, which, according to the 

 degree of fusion, may be a single tube or divided by a 

 partition. Moreover, in Bennett's kangaroo, the caecum 

 opens independently into the cloaca between the apertures 

 of the distal portions of the vaginae, and forms the so-called 

 third vagina. In Perameles, although such a median 

 passage does not exist in the young female, it is formed by 

 a process of rupture at the period of parturition. The true 

 vaginas are apparently too narrow to allow for the passage of 

 the embryos. 



The allantois in Perameles, as already seen, forms u true allantoic 

 placenta ; in Phascolarctos it fuses with the subzonal membrane, 

 becomes highly vascular, and functions as an embryonic respiratory 

 organ, but does not unite with the uterine wall ; in all other Mar- 

 supials, so far as is known, it is small, only projects slightly into the 

 extra-embryonic body cavity, and is apparently functionless. Accord- 

 ing to Hill, the condition seen in Perameles is primitive, and the other 

 Marsupials show degeneration. The wall of the umbilical vesicle or 

 yolk-sac is highly vascular, and may unite with the uterine wall to form 

 a yolk-sac placenta. 



The gestation is short, only lasting a fortnight in the 

 opossum, about five weeks in the kangaroo ; whereas that of 

 the mare, for instance, is about eleven months. Except in 

 some opossums, there is a marsupial pouch, usually with a 

 forward-directed aperture. Within this pouch are the teats, 

 and here the delicate young are nurtured after birth. As 

 they are unable to suck, the milk is forced down their 

 throat, the mammary gland being compressed by the 

 cremaster muscle which covers it. Vague vestiges of a 

 marsupium are said to be visible in some Placentals. 



Classification of Marsupials. — The Marsupials are divided 

 into two sub-orders, each of which contains four families. The two 

 sets are defined by the characters of the teeth, which are, of course, 

 adapted to habit. In the members of the first sub-order the incisors 



are numerous (not less than -), small, and almost equal in size ; while 



the canines are large, and the molars furnished with sharp cusps. The 

 whole dentition presents a striking resemblance to that of the Eutherian 

 Carnivores. To this group the name Polyprotodontia is applied, and 

 the forms included in it are typically carnivorous or insectivorous. The 

 csecum is absent or very small. 



